The present invention relates to an apparatus for uninterruptedly supplying power including an electrical machine including a rotor and being operable as a motor or as a generator, the electrical machine being connected to a load to be supplied with alternating current without a converter having a variable feed frequency being arranged in between, a flywheel being coupled to the rotor by coupling units having a variable ratio of transmission, and control units for the transmission of the coupling units, the control units at least in a certain range of the number of rotations of the flywheel keeping the number of rotations of the electrical machine being operated as a generator constant by the kinetic energy being stored in the flywheel.
Apparatuses for uninterruptedly supplying power serve to compensate especially short-term failures of a power network. Generally and also hereinafter, they are often called UPS systems. Usually, the power network is used to supply a load with alternating current. It is a known concept to operate an electrical machine as a motor in addition to the load by means of the power network. The electrical machine affects a flywheel in a way to reach a certain number of rotations, and it keeps the flywheel at this number of rotations. In case the network fails, the kinetic energy being stored in the flywheel may be used to operate the electrical machine as a generator to supply the load with alternating current. Consequently, the kinetic energy of the flywheel and its number of rotations decrease. When the flywheel is fixedly coupled to the generator, a converter having a variable feed frequency has to be arranged between the generator and the load to feed the load with alternating current at a constant frequency. Especially in the medium voltage range of approximately 10.000 volt, such a converter, as well as a d.c./a.c. converter that would have to be arranged after a direct current machine as electrical machine, is a complex unit when it is to be designed for greater capacities, meaning for greater currents. However, even if it is designed for greater capacities, it is extremely sensitive to short circuit currents.
It is known to design an apparatus for uninterruptedly supplying power in which the flywheel is not fixedly coupled to the rotor of the electrical machine, but instead by coupling units having a variable transmission. In a concrete known apparatus for uninterruptedly supplying power by the firm HOLEC/HITEC, the coupling units include an electromagnetic clutch being arranged between the electrical machine and the flywheel. The electromagnetic clutch enables the flywheel to decelerate without the number of rotations of the electrical machine being operated as a generator decreasing. In case of a simple structure of the electromagnetic clutch, this is possible as long as the number of rotations of the flywheel is more than the desired constant number of rotations of the generator. In this case, it is necessary that the flywheel be accelerated by the electrical machine being operated as a motor via a different torque transmission path to reach a greater number of rotations than the number of rotations of the electrical machine. When the electromagnetic clutch is desired to also allow for an increasing transmission of the respective driving feed number of rotations, the design of the electromagnetic clutch and the design of the required control units is especially complicated.
It is also known from prior art UPS systems to provide a combustion engine for longer-term failures of the network to operate the electrical machine as a generator when longer-term power failures have to be compensated. The rotor of the electrical machine is connected to the generator by an overrunning clutch or a by a switchable clutch. When no converter having a variable feed frequency is arranged after the electrical machine, the number of rotations of the combustion engine must have already reached the number of rotations of the electrical machine before it may be coupled to the electrical machine. Then, the number of rotations has to be kept constant.
An electrical generator arrangement is known from U.S. Pat. No. 4,278,928. A differential transmission in the form of a planetary transmission is arranged to precede the input shaft of an electrical generator. The input shaft of the generator is connected to the sun wheel of the planetary transmission. The cage rotor of the planets of the planetary transmission is connected to the input shaft of the entire electrical generator arrangement. The gear rim of the planetary transmission may be driven at different numbers of rotations by a hydraulic drive to vary the ratio of transmission of the planetary transmission such that the number of rotations of the generator is kept constant even when the number of rotations at the input shaft of the entire electrical generator arrangement changes. The hydraulic medium for the hydraulic drive is provided by pumps that are driven by the input shaft of the generator rotating at a constant number of rotations or by a different element of the generator.
The coupling units include a differential transmission including three input/output shafts and that the control units include an electrical auxiliary machine being operable as a motor and a controllable brake, the electrical auxiliary machine and the controllable brake engaging the third input/output shaft of the differential transmission, wherein the control units are designed in a way that the auxiliary machine may also be operated as a motor to reach the desired final number of rotations of the flywheel when the electrical machine is operated as a motor.
In the novel UPS system, the flywheel is connected to the rotor of the electrical machine by a differential transmission. A differential transmission means a mechanical transmission including three input/output shafts of which at least two are not coupled to one another. The differential transmission is used as a transmission having a variable ratio of transmission by an electrical auxiliary machine being operable as a motor and a controllable brake both engaging the third input/output shaft of the differential transmission. By means of the auxiliary machine, additional kinetic energy is stored in the flywheel during operation of the electrical machine as a motor. During operation of the electrical machine as a generator, the number of rotations of the electrical machine may be kept constant by means of the controllable brake at least until the number of rotations of the third input/output shaft of the differential transmission has decreased to reach zero.
Typically, the capability of the auxiliary machine in the novel UPS system is substantially less than the capability of the electrical machine.
For example, the differential transmission of the novel UPS system may be a mechanical differential transmission or a planetary transmission.
In a preferred embodiment of the novel UPS system, the differential transmission is designed as a planetary transmission including a sun wheel, a cage rotor carrying planetary wheels and a gear rim. The sun wheel is fixedly connected to the input/output shaft leading to the flywheel and the cage rotor is fixedly connected to the input/output shaft leading to the electrical machine. The gear rim is coupled to the input/output shaft leading to the auxiliary machine at a fixed ratio of transmission. In case of this arrangement, the planetary transmission serves for a transmission of the number of rotations of the motor into a greater transmission of the number of rotations of the flywheel to store as much kinetic energy as possible in a flywheel having a given moment of inertia. This transmission is further increased with the electrical auxiliary machine by driving the gear rim in an opposite direction. During the use of the energy of the flywheel, the electrical energy that has been produced by the auxiliary machine to increase the transmission of the planetary transmission is recovered. The electrical energy is recovered except a small portion thereof that has been converted to heat.
The controllable brake may be an additional mechanical brake. However, the electrical auxiliary machine may instead be designed to be operable as an electromagnetic brake. The brake consumes the power that has been produced in the auxiliary machine or it may be used as emergency power.
In the simplest case, the electrical auxiliary machine is an asynchronous machine that is directly connected to be power network by the control units when it is operated as a motor. An asynchronous machine as auxiliary machine is to be combined with an additional controllable brake. When the number of rotations of the third input/output shaft of the differential transmission has been reduced to reach zeroxe2x80x94while the asynchronous machine typically has been switched offxe2x80x94the control units may connect the asynchronous machine to the electrical machine being operated as a generator with a polarity being inverted compared to the network operational mode to accelerate the third input/output shaft of the differential transmission in an opposite sense of rotation. The exact control of the number of rotations for keeping constant and the number of rotations of the electrical machine is again achieved by means of the mechanical brake.
When the electrical auxiliary machine is a synchronous machine that is connectable to the electrical machine and to the load by a converter, the auxiliary machine may be operated as the brake or at least to provide a portion of the braking effort. The power that is produced in the auxiliary machine while braking may be fed to the load by the converter such that the electrical energy is not lost. Furthermore, the use of the synchronous machine with the converter makes it possible to theoretically use the kinetic energy of the flywheel until its number of rotations has reached zero. This means that the kinetic energy of the flywheel may be fully used. In the lower range of numbers of rotations of the flywheel, it is necessary to operate the electrical auxiliary machine as a motor at a sense of rotation being opposite compared to the one in the network operational mode. This serves to maintain the number of rotations of the rotor of the electrical machine. The electrical machine being operated as a motor by the electrical auxiliary machine provides the required energy. In this way, the electrical auxiliary machine compensates the energy balance except the power loss. Only a portion of the total power flows through the converter being located between the electrical auxiliary machine and the electrical machine and the load, respectively. Consequently, the converter has a substantially simpler design compared to one used for an electrical machine that is operated at a variable number of rotations.
The electrical auxiliary machine may also be designed as a direct current machine that is connectable to the electrical machine and to the load by a converter. In this case, the same operational modes are possible as it is the case with an asynchronous machine including a converter.
Usually, the electrical machine of the novel UPS system is a synchronous machine having a simple design at low costs although it is very powerful.
In addition to the flywheel being provided to compensate short-term power failures with its kinetic energy, the rotor may be drivable by a combustion engine to compensate long-term power failures. A clutch being switchable by the control units may be arranged between the rotor and the combustion engine.
It is especially preferred to arrange a differential transmission between the rotor and the combustion engine. The differential transmission is provided in addition or alternatively to the switchable clutch. The third input/output shaft of the differential transmission is coupled to another electrical auxiliary machine of the control units. The additional auxiliary machine serves to compensate differences concerning numbers of rotations of the rotor with respect to the combustion engine, especially when the combustion engine has not yet reached its final number of rotations. The final number of rotations of the combustion engine is either approximately identical to the number of rotations of the rotor, or it is related thereto with a fixed ratio.
In a further developed embodiment, the combustion engine does not require a starter motor since the differential transmission is arranged between the combustion engine and the rotor. The energy being necessary to start the combustion engine is then provided by the electrical machine in combination with the additional auxiliary machine and, finally, by the flywheel.
However, it is also possible to arrange an overrunning clutch between the rotor and the combustion engine. In this case, a separate starter motor for the combustion engine is required.
In the novel UPS system, the running up of the electrical machine into a range of numbers of rotations in which it is operable as a synchronous motor may be realized by means of the electrical auxiliary machine without an additional motor. The flywheel may be kept still. Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and the detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention, as defined by the claims.